Riser system for a slacked moored hull floating unit

ABSTRACT

A riser tensioning system with an individual riser unit on an oil or gas platform. The platform has a traveling trolley structure with at least one trolley bearing and a centralizer. The trolley bearing is coupled to at least one guide rail configured to allow vertical movement of said traveling trolley structure. The traveling trolley structure is coupled to a riser collar configured to support a top of the riser. The riser tensioning system has at least one cylinder coupled to the traveling trolley structure on one end and secured on an opposite end such that the cylinder is adapted to push or pull the traveling trolley structure vertically.

CLAIM OF PRIORITY OR CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority to Provisional Patent Application Ser. No. 61/388,825, filed Oct. 1, 2010, the contents of which are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to a riser tensioning system for a floating oil and gas platform, in particular to a slacked moored floating body where the heave motions are significantly larger than the heave motions in tension leg platforms (TLPs) and high motion suppression units, like spars and optimized multicolumn semisubmersibles hulls with or without damping plates and mechanisms.

BACKGROUND OF THE INVENTION

As oil and gas production begins to take place in deeper waters, a shift from fixed platforms to floating platforms has become necessary. Oil and gas production now occurs from tension leg platforms (TLPs), deep draft floaters (DDFs), multicolumn semisubmersible hulls, and floating spars.

During production, vertical pipelines bring the oil or gas up from the sea bed to the floating hull for processing. These vertical pipelines are known as ‘risers.’ The risers must be held in constant tension against the vertical motions, or heaves, of the floating hulls. Riser tensioning systems have been developed to maintain a tension level at a predetermined desired tension and/or within a predetermined desired tension level range.

The prior art includes direct action riser tensioning systems for exploration rigs and TLPs. In these systems, risers are supported by direct acting hydraulic tensioners that act in a pull motion with the exploration rig or TLP. The stroke compensation needs for TLPs are only in the range of up to 3-5 feet. A DP drill ship having a similar system can compensate more than 60 ft of motion.

Direct action riser tensioning systems use the same system components and configuration for the entire production system, which has several disadvantages. Among these disadvantages are the x-mas trees experience large translations and rotations within the wellbay volume. Additionally, when the system is in the upstroke condition, there is the risk of clashing by the x-mas trees and/or flowlines. In these systems, the x-mas trees are positioned on large “levers,” which travel in distance from the x-mas trees to the riser centralizers located at the tree (lower) deck.

Direct action riser tensioning systems also pose technical challenges with respect to placing restricting elements into the riser arrangement. One example of this problem is seen when a blowout preventer (BOP) need to replace a x-mas tree during drilling. Additionally, a sub cellar deck may be required in the wave zone. Another problem with these systems is the tensioning cylinders are mostly situated below deck, where they are exposed to salt sediments and corrosion in the wave zone, which complicates inspection and maintenance.

Other forms of tensioning systems in the prior art include RAM systems. These systems provide for stroke lengths of 28-29 feet, and a riser support arrangement that eliminates the rotation of the riser and x-mas tree. In these systems, the compensation of the x-mas tree is a mostly vertical displacement.

One problem with these systems is they require keel guides and other guides in the hull structure. Keel guides are problematic with respect to riser fatigue, friction and stiction forces. Additionally, there are no inventions that allow for the keel joints to travel 40 feet.

Another problem with RAM systems is the floating platform must be highly optimized with regards to motion suppression. This is why RAM systems are typically implemented on floating device such as a spar with a 5-600 foot draft or a semisubmersible hull.

All of the motion suppressors in RAM systems have certain drawbacks in the project execution phase including extreme drafts, large displacements and costly integration issues such as offshore lifting and the integration of other facilities, in the case of the spar.

In the past, it was assumed that a RAM design that could compensate up to 40 feet would require massive structures because of the guiding structures and the storage arrangements of air pressure vessels.

Another problem with RAM systems is due to the problems caused by the pushing RAM cylinders. These cylinders tend to push the x-mas trees upwards in the wellbay arrangement, which necessitates a similar upward movement of the drill floor. These counter balancing motions create more weights, wind area, payload restrictions and operative restrictions. These problems may be avoided by moving the RAM system below the deck in the wave zone, but this is not desirable due to the effect of the wave impact and the corrosive environment.

Another type of riser tensioning system is a traveling deck compensation system. The main difference between traveling deck compensation systems and both direct acting and RAM tensioning systems is that in traveling deck compensation systems, the rotation of the x-mas tree is controlled by a centralizer, located immediately below the x-mas tree. When the upper riser imposes a bending angle on the x-mas tree, the system allows the x-mas tree to rotate slightly around this pivot point. However, since the angle as well as the lever is small (the lever is merely the height of the x-mas tree itself plus the stroke of the first tier), the rotational motion envelope of the x-mas tree is moderate and the motion is essentially vertical.

A problem with traveling deck compensation systems is these systems must be a two tier system. A system without the first tier compensation would result in a loss of ability to compensate for a delta displacement taking place across the deck from a tilt or heel motion.

Another type of riser tensioning system is a lever action system. These systems include systems for a TLP where the tension action is created by a hinged lever that amplifies the tensioning device's stroke, creating a larger stroke on the support for the x-mas-tree.

The compensation track of the riser support is a rotation induced translation of the x-mas tree support following a circular motion around the hinged center. Upon completion of each well and production riser in the center slot, the riser is shifted to the dedicated slot by means of a traveling bridge crane in the well bay.

Accordingly, there is a need for a riser tensioning system that supports larger heave motions and provides a system of compensation for individual wells. The present invention discloses a riser tensioning system that remedies the deficiencies of the prior art.

SUMMARY OF THE INVENTION

The present invention is a riser tensioning system with an individual riser unit on an oil or gas platform. Any suitable tension level or tension level range can be utilized in accordance with the present invention, including those which are known in the art. In one embodiment of the present invention, the riser tensioning system includes a traveling trolley structure having at least one trolley bearing and a centralizer. The system also includes at least one cylinder coupled to the traveling trolley structure on one end and secured on an opposite end, wherein the cylinder is adapted to push or pull said traveling trolley structure vertically. The system also includes at least one guide rail coupled to the trolley bearing, configured to allow vertical movement of the traveling trolley structure. Lastly, the system includes a riser collar coupled to the traveling trolley structure and configured to support a top of the individual riser.

According to another embodiment of the present invention, a method is provided for applying tension to an individual riser unit on an oil or gas platform, where the platform has a riser with a x-mas tree and a traveling trolley structure with at least one trolley bearing and a centralizer. The method comprises the step of attaching at least one cylinder to the traveling trolley structure and securing the other end of the cylinder, such that actuation of the cylinder effects vertical movement of the traveling trolley structure.

According to another embodiment of the present invention, a riser tensioning system is provided to an oil or gas platform with an individual riser unit. The platform has a traveling trolley structure with at least one trolley bearing and a centralizer. The trolley bearing is coupled to at least one guide rail configured to allow vertical movement of said traveling trolley structure. The traveling trolley structure is coupled to a riser collar configured to support a top of the riser. The riser tensioning system comprises at least one cylinder coupled to the traveling trolley structure on one end and secured on an opposite end such that the cylinder is adapted to push or pull the traveling trolley structure vertically.

The present invention improves upon past traveling deck compensation systems such as the system in U.S. Pat. No. 6,691,784. The system in U.S. Pat. No. 6,691,784 utilizes a stroke compensation of heave and combined roll and pitch displacement for an array of risers suspended on a moving table x-mas tree deck from which the risers are supported and configured in a suitable slot pattern. In contrast, certain embodiments of the present invention are based on a system of compensation for individual wells. In certain embodiments of this invention, the system is split such that each riser is served by a dedicated traveling tensioning assembly. Therefore, the delta stroke is compensated for by the individual riser system.

Thus, the present invention may include a two tier compensation system and a single tier system where the compensation is performed 100% by ‘the single unit x-mas tree deck,’ also known as ‘the x-mas tree and riser support trolley’ or simply ‘the trolley.’

The present invention will allow the vessel motions effect on the riser tensioning and stroke compensation to be dealt with through the rotation of the riser support and the X-mas tree at the point of suspension.

Such a structure allows for the elimination of a keel guide sliding bearing, the use of sliding bearings at another elevation deck or the use of other means to minimize rotation or translation of the X-mas tree within the wellbay arrangement, a well recognized challenge when dealing with a number of risers in compact arrays.

The structure of the present invention addresses the problem of equipment clashing into other equipment within the wellbay arrangement. The present invention also addresses the problem posed by riser keel joint contact areas, which are a concern for both high local stresses as well as long term fatigue. In the present invention, there is no keel joint. Therefore, the riser will act more like a second order structural element with a close to nil moment at its support, dominated by pure tensional stress at the cost of some rotational displacement of the x-mas tree where the riser support will be the x-mas tree pivoting point relative to the riser.

Other keel guide problems are eliminated by the present invention. In other systems, the need for buoyancy elements and strake elements made installation of the riser complicated, as all of the components had to pass through the keel guide arrangement when it was deployed. The present invention eliminates this problem, as there will be no clearance or clash problem during deployment other than problems from other suspended risers.

The present invention also be designed to allow for the riser/x-mas tree to be affixed to the riser support at a location where the fixation forces are all transferred to the riser support trolley by means of a so-called stress joint.

Furthermore, in contrast to lever based systems, such as the hinged type lever tensioning mechanism is U.S. Pat. No. 6,585,455, the present invention utilizes a direct action tensioning means where the riser and the tensioning device may have about a linear 1:1 vertical translation motion ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

While the present invention may be embodied in many different forms, a number of illustrative embodiments are described herein with reference to the accompanying figures. The skilled person should understand that the present disclosure is to be considered as providing examples of the principles of the invention and such examples are not intended to limit the invention to the preferred embodiment described herein and/or illustrated herein.

FIG. 1 is a side view of a push type guided trolley riser support system, according to an embodiment of the present invention.

FIG. 2 is a side view of a push type guided trolley box structure riser support system, according to another embodiment of the present invention.

FIG. 3 is a side view of a pull type guided trolley beam riser support system, according to another embodiment of the present invention.

FIG. 4 is a side view of a pull type guided trolley box structure riser support system, according to another embodiment of the present invention.

FIG. 5 is a side view of a single sided push type guided trolley box/cantilevered structure riser support system, according to another embodiment of the present invention.

FIG. 6 is a side view of a single sided pull type guided trolley box/cantilevered structure support system, according to another embodiment of the present invention.

FIG. 7 is a side view of with an arrangement capable of 40 ft stroke capacity. The system is a pulling trolley riser tensioning system for a dry tree semi, according to an embodiment of the present invention.

FIG. 8 is a side view of an arrangement of riser tensioning system for a dry tree semisubmersible shown under extreme heel angle with riser in maximum up-stroke position, according to an embodiment of the present invention.

FIG. 9 is a side view of an arrangement of riser tensioning system array for a dry tree semisubmersible, according to an embodiment of the present invention. The max down stroke comprises the left side of the figure and the max up-stroke comprises the right side of the figure.

FIG. 10 is a close up view of an arrangement of a riser tensioning system for a dry tree semi, according to an embodiment of the present invention.

FIG. 11 is a side view of a dry tree semisubmersible with pull type trolley riser tension system and space for surface BOP and telescopic joint above the wellbay, according to an embodiment of the present invention.

FIG. 12 is a dry tree semisubmersible with pull type trolley riser tension system in response to extreme damage heel condition (water ingress in hull), according to an embodiment of the present invention.

FIG. 13 is a side view of a system operating onboard a monohull floater with moonpool, according to an embodiment of the present invention.

FIG. 14 is a side view of a system operating outboard of a monohull floater, according to an embodiment of the present invention.

FIG. 15 is a possible version of prior art, U.S. Pat. No. 6,691,784.

FIG. 16 is a tensioning via wire sheave system, according to an embodiment of the present invention.

FIG. 17 is a two perspective view of a single well two tier based system with wire/sheave arrangements, according to an embodiment of the present invention.

FIG. 18 is a three perspective view of a single well single tier based tensioning system, according to an embodiment of the present invention.

FIG. 19 is a sectional perspective view of a typical wellbay section of a single well compensation two tier system, according to an embodiment of the present invention.

FIG. 20 is an upward view of a typical wellbay arrangement with a single well compensation two tier system, according to an embodiment of the present invention.

FIG. 21 is a side view of an individual riser with a trolley structure, according to an embodiment of the present invention.

FIG. 22 is a side view of an individual riser with a hydraulic cylinder system, according to an embodiment of the present invention.

FIG. 23 is an overhead view of an individual riser with a trolley structure, according to an embodiment of the present invention.

FIG. 24 is an overhead view of an individual riser with a trolley structure, according to an embodiment of the present invention.

FIG. 25 is a rear view of an individual riser with a trolley structure, according to an embodiment of the present invention.

FIG. 26 is a rear view of an individual riser with a trolley structure, according to an embodiment of the present invention.

FIG. 27 is a side view of an individual riser with a trolley structure under deflection and stress, according to an embodiment of the present invention.

FIG. 28 is a side view of an individual riser with a trolley structure under deflection and stress, according to an embodiment of the present invention.

FIG. 29 is a side view of an individual riser with a trolley structure, according to an embodiment of the present invention.

FIG. 30 is a front view of an individual riser with a trolley structure, according to an embodiment of the present invention.

FIG. 31 is an overhead view of an individual riser with a trolley structure, according to an embodiment of the present invention.

FIG. 32 is a slanted front view of an individual riser with a trolley structure, according to an embodiment of the present invention.

FIG. 33 is an overhead side view of the trolley structure, according to an embodiment of the present invention.

FIG. 34 is a front view of a wellbay structure, according to an embodiment of the present invention.

FIG. 35 is a front view of a group of risers, according to an embodiment of the present invention.

FIG. 36 is a side view of an individual production riser, according to an embodiment of the present invention.

FIG. 37 is a front view of a wellbay structure, according to an embodiment of the present invention.

FIG. 38 is a side view of a wellbay structure, according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

One possible system allows each riser or x-mas tree in a well/risers array to be compensated individually. The appearance of this system could be similar to traveling deck compensation riser systems except the traveling x-mas deck or “trolley” would only carry one riser/x-mas tree. This system can be a two tier system or a single tier system. As opposed to past systems, the single tier system will be a feasible approach as the delta stroke across the wellbay can be compensated automatically by the individual riser systems. Seven different alternatives of the invention follow:

Alternative I

FIG. 1 shows a push type guided trolley riser support system 100. A x-mas tree 101 on top of the riser is supported by a traveling trolley structure 108, which in this case is a beam structure within a trolley box 107. The traveling trolley structure is controlled by a set of push hydraulic cylinders 105(a), 105(b). Of course, a controller (not shown) can be provided on or remote to the cylinder to control actuation of the cylinder. The traveling trolley structure 108 is guided by bearings 102(a), 102(b) that run on the face of guide structures 104(a), 104(b). The riser 103 pivots about the centralizer 110 located in the trolley beam 108. The riser 103 will remain grossly vertical when the vessel is subject to heel, and the riser system must be designed to have sufficient clearances 109 at the bottom of the guide structures 104(a), 104(b).

In the drilling mode a set of centralizers 106 will be placed at the lower part of the arrangement to allow a BOP (substituting the x/mas tree) to be supported while always remaining vertical on top of the traveling trolley beam as the BOP must line up with a telescopic joint and the drill floor center/diverter.

There are several requirements for certain embodiments of Alternative I. One requirement is the system must control the individual cylinder's actions in order to control the leveling position of the trolley beam. Another requirement is the system must compensate for clearance issues between the lower part of the riser and the adjacent structures. Additionally, the drilling condition may require a sliding bearing type centralizer at the bottom of the arrangement. In this setup, a larger portion of the riser in contact with the centralizer may attract high loading from the centralizer onto the riser along the “stroke” part. This may result in heavy walls and or more complicated solutions like double walls over the stroke length. Also, the lower centralizer may demand a spider deck below the lower deck. Finally, the system may bring the drill floor upwards as stack up of the push cylinders will add operational height. This problem can be mitigated by allowing the cylinders to protrude below the lower deck which will expose them to wave impacts.

Alternative II

FIG. 2 shows a push type guided trolley box structure riser support system 200. This system operates somewhat similarly to Alternative I. There are differences, however, as the traveling trolley structure 108 is a box structure 212 arranged with bearings 210(a), 210(b) that will allow the box 212 to remain stable on its track, while reducing the possibility for run-away cylinder actions. Eccentric loading will see responses in the form of force couples acting at the bearings.

Another difference is the trolley box structure 212 may, in its drilling mode, have the lower centralizer 213 fit in the lower part of the box. The lower centralizer 213, together with the upper or main operational centralizer 110, will keep the riser 103 in a position that is at least close to vertical at the wellhead flange, so that a BOP can be kept in a vertical position. Since the lower centralizer 213 travels with the riser 103 it remains static relative to the riser 103 and supports the riser 103 in a fixed position. This allows for the use of a dedicated metallic riser flex joint element to control the stress in this location. Depending on configuration and drilling operations (wet or surface BOP), this arrangement may only be required for the drilling riser.

Alternative III

FIG. 3 shows a pull type guided trolley beam riser support system 300 that is similar to Alternative I.

One main difference is stack up height resulting from hydraulic cylinders 305(a), 305(b) can be avoided. The x-mas tree 101 may travel all the way down to the lower deck. Another difference is the clearance 109 from riser 103 to adjacent structures is improved as the cylinders 305(a), 305(b) are out of the way. Yet another difference is in the drilling mode, the lower centralizer 110 may require a spider deck below the lower deck.

Alternative IV

FIG. 4 shows a pull type guided trolley box structure riser support system 400 that is similar to Alternative III. The first difference is Alternative IV is more favorable with respect to the riser 103 position and clearance 109 as the cylinders 405(a), 405(b) are out of the way. Another difference is the guide rails 104(a), 104(b) may have to be extended partly below the lower deck based on the height of the trolley box 107 when the x-mas tree 101 is in the lower position.

Alternative V

FIG. 5 shows a single sided push type guided trolley box/cantilevered structure riser support system 500 that is similar to Alternative II. The first of many differences is the traveling trolley structure 512 runs on a guide system 504, which is connected by traveling trolley bearings 502 on only one side. A second difference is the push cylinders 505 act eccentrically on the trolley 512, which are only on one side. A third difference is the eccentric moment between cylinder forces and riser tension may be taken as force couples in the supports. A fourth difference is the riser support may be shifted outwards on the box/cantilever 107 in order to create the desired clearance 109 between riser 103 and adjacent structure. A last difference is the system allows favorable no-obstruction arrangements for flexible flowline hose loops in the space outside the box/cantilever structure.

Alternative VI

FIG. 6 shows a single sided pull type guided trolley box/cantilevered structure riser support system 600 that has many of the features in Alternatives I through V. There are several advantages to Alternative VI, including the minimum stack up height and maximum clearance 106 between riser 103 and adjacent structures. This allows for large heel angles even in the maximum upstroke position, even when the guide beams 504 are protruding slightly below the lower deck into the wave zone. Another advantage is the second centralizer 612 allows drilling with an upright BOP without introduction of a spider deck below the lower deck.

FIGS. 7-9 show this system for a dry tree semisubmersible hull from different perspectives.

FIG. 10 shows an alternative view of this system where the vessel heel is tilted 8-10 degrees. This condition is typical for an accidental damage condition and is accompanied by water ingress in the hull. In this condition, the riser clearance 1009 is in a critical condition and the trolley is in a max upstroke position.

FIGS. 11 and 12 show this system with a space for a surface BOP and telescopic joint 1101 above the riser area 1102. In FIG. 12, the system has responded to extreme damage heel condition.

FIG. 13 shows this system operating onboard a monohull floater with moonpool 1302. The tensioner assembly is upright in a work over/drilling position. The guide structures 1303 are inclined in order for risers to stay clear of the moonpool 1302, hull structure, and guides when the hull is subject to a roll or pitch motion.

FIG. 14 shows this system operating outboard of a monohull floater. The guide structures 1402 are inclined in order for risers to stay clear of ship's side and guide structures 1402 are controlled by a jacking/locking mechanism 1401.

Alternative VII

FIG. 15 shows an alternative based on wire sheave pulling arrangements 1500 instead of a direct action system. Alternative VII is one version of a two tier version of a riser tensioning system. One feature here is that through the use of wire and sheave arrangement the large stroke tensioning cylinders 1502 can be placed a distance away from the compensated TLP deck 1501. One can also place these cylinders 1502 in separate controlled environment “machinery spaces” 1503. This can save a great deal in cost and maintenance, as it places the cylinders away from the salty corrosive environment of the open wellbay itself.

The same system is shown schematically in FIG. 16. Note the placement of large stroke cylinders in the “machinery spaces” 1503.

FIGS. 17-19 show different perspectives of a two tier version of Alternative VII 1705. The first tier 1704 is designed to operate alone 99% of the time to avoid too much wire/sheave wear. The second tier 1701 can become a “storm system.” The second long stroke tensioning system is enclosed in machinery space 1703, where only wires can penetrate the space 1702.

FIG. 20 shows a one-tier version of Alternative VII. All of the motions are taken by the wire/sheave tensioner system 2001. Additionally, the stroke tensioning system is enclosed in machinery space 2003, where only wires can penetrate the space 2002.

FIGS. 21-26 show detailed perspectives of an embodiment of the present invention. In these systems, the guide rail 2101 is attached to the deck beam 2102 both above and below the structure. Likewise, the hydraulic cylinder barrel 2103 is attached to the deck beam 2102. The trolley structure 2104 is attached to the guide rails 2101 by a sliding sleeve 2105. The riser 2106 extends through the hollow area 2107 of the trolley structure 2104. Atop the trolley structure 2104 is both the riser collar 2108 and x-mas tree 2109.

FIGS. 27 and 28 show this system under deflection and stress caused by wave action, current, platform offset, or platform heel and trim 2703. During this stress, the spherical bearing 2701 takes vertical force and horizontal shear load from the riser and transfers it to the floater through the trolley structure 2104. Also during this stress, the shear and moment is transferred from the riser to the floater hull through the trolley structure 2802. There is also a stress joint 2801 that controls the riser curvature and stress. In the system, the production riser 2106 can be with or without centralizing bearings or a keel joint 2702.

FIGS. 29-32 show detailed perspectives of another embodiment of the present invention. The components work the same as in the one-tier system with the addition of a second tier comprised of compressed air bottles and oil pressure vessels 2901. Atop the second tier are shackles 2902.

FIG. 33 shows a detailed perspective of a trolley structure 3300. The trolley structure has a horizontal bearing frame 3301 and contains a hollow 3302 conductor pipe 3303.

FIG. 34 shows the riser tensioning system of the present invention within a wellbay structure 3401. The wellbay structure 3401 extends from skid beams 3402.

FIG. 35 shows an alternative embodiment of the present invention in a wellbay structure 3401. This system has flexible hoses 3502 and is connected to shackles 3501. The wellbay structure 3401 also has a below deck support structure for guide rails 3503.

FIG. 36 shows a detailed perspective of a single riser unit 3600 in another embodiment of the present invention. In this embodiment, the riser 2106 is fit into a conductor pipe 3605. The conductor pipe 3605 is itself fit through a horizontal bearing frame 3601 as well as the deck beam 3606. Above the deck beam 3606 is a tensioner cassette 3604 with attached hydraulic piston rods 3606. A x-mas tree 2109 rests upon the riser 2106 with an attached flexible hose 3502.

FIGS. 37 and 38 show this system as fit within a wellbay structure 3401.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

One specific embodiment features a suspended platform (trolley), with the supports that keep the platform suspended offset to one side, rather than symmetrical. Advantages of this arrangement include specifically reduced required heights, less strain on connections, more open space for maintaining both the described suspension system and the suspended pieces. The eccentric pull that is applied on the system (trolley), relative to the riser tension vector is an important factor since the force being exerted on the trolley is not simply a vertical force, as the offset nature of the supports creates a horizontal force component as well.

This specific embodiment offers several advantages including reducing height requirements for drill floor elevation, especially when using “pull” type cylinders. Another advantage is the embodiment allows for pulling cylinders to be arranged so that they are structurally isolated from imposed deflections/distortions, which reduces wear on packers and connection details.

This embodiment also provides a clean arrangement (clear “air space” above and around the x-mas trees) with lots of space for production flexible house routing and general access for maintenance or repair.

This embodiment also offers many protective advantages in the event of a potential problem. The embodiment provides protection around the cylinders to protect them from dropped objects, explosions or fire. Additionally, in this embodiment, the loss of one operational cylinder will not affect operation as it tolerates the eccentric moment in the design (symmetrical systems may need to take out pairs of cylinders due to an imbalance that is not tolerated).

Another advantage of this embodiment is it may allow for rotation of the x-mas tree from imposed riser flexure in systems with a keel joint (similar to typical spar arrangements) or in systems without a keel joint (similar to typical TLP arrangement).

Another advantage to this system is it may be configured without rotation of the x-mas tree where the moment is absorbed by the trolley structure via a riser stress joint to control the riser stress; this is a “fully fetched” no keel joint dry tree semi tensioner.

This embodiment offers repair advantages too, as all or most components can be inspected and repaired at deck elevation. For example, the cylinders may be changed out by pulling them from the top deck, which provides for reduced clash opportunities with delicate equipment.

Yet another advantage is none of the tensioning system hydraulic piping will run in the wellbay area. Hydraulic oil and accumulator gas connecting piping and accumulator pressure vessels will be routed from outside the wellbay area to active cylinders that also are away from the HP hydro carbon containing components. This will protect the tensioning system functionality in case of dropped objects or clashing of equipment in the wellbay area.

Finally, this embodiment offers many weight saving and cost saving benefits.

The preferred embodiment may include several distinct, non-limiting features. One potential feature is a riser top tensioning device based on passive hydro pneumatic cylinders where the push or pull force is conveyed to a vertically guided and rotation restrained traveling trolley or rigid sliding structure supporting a collar co-axially oriented with the riser, said collar supporting the top of the riser and equipment thereto attached.

The optionally vertically acting and oriented cylinders, may be configured in an asymmetric pattern about at least one axis in the horizontal plane relative to said collar constituting a resultant force that balances the riser vertical load in orientation and magnitude but may have an eccentricity relative to the riser vertical load consisting of dominantly riser weight and a selected percentage of over pull.

The eccentricity may create static and dynamic moment imbalances occurring largely about various axes located in the horizontal plane.

The cylinders may be arranged so their vertical acting resultant occurs in the plane of the rails/bearing, which constitutes the maximum desired eccentricity relative to the riser tension, as anything beyond this placement of cylinders may increase the loads on the trolleys rotational restraining elements.

The cylinders may be arranged such that the eccentricity is reduced to a minimum, as a trade off for access and operation to the x-mas tree or BOP, thus reducing the loads on the trolleys rotational restraining elements.

The moment imbalances and/or shear and moments carried by the riser pipe itself may be controlled by the rigidity of the guided trolley or sliding structure and transferred to surrounding structures of the floating installation by means of a substantially vertically arranged guide rail system resulting in a controlled vertical direction horizontally guided translatory motion of the collar and riser top.

The vertical guide rails can vary from one rail with rotational restrained raceways or sliding surfaces to a number of rails. Additionally, the collar can feature a spherical bearing or flexible pads thus allowing the riser top to rotate with close to zero or designed end fixation stiffness.

The collar can feature an arrangement where the riser top is fully fixed to the collar as well as the trolley structure in the form of a flex joint or stress joint where the curvature and stress of the riser is controlled at all times.

The riser can run through a set of one or more centralizer bearings acting on the riser, fixed to the floater hull in the form of a keel guide normally at the bottom entry of the floater or at another elevation clear and below the tensioner trolley motions.

The collar and the trolley structure may be arranged so that after removing the riser and collar, equipment can be run through a circular or other shape light opening vertically through the trolley structure allowing for the lowering and pulling down of equipment to the seafloor for equipment of all dimensions that can pass through the drill floor rotary opening.

The collar structure may be expanded vertically to become a hollow structural pipe (“conductor pipe”) that allows equipment to be run through the conductor pipes light opening of all sizes that can pass through the drill floor rotary opening.

The riser may be conveyed directly into the structural collar/conductor pipe without being guided by any keel joint/guide or otherwise where the riser forces are conveyed directly into the structural pipe and trolley/guide structure.

The riser may be a drilling, work over or production riser.

The tensioning device may be arranged in an array or grid pattern similar to a TLP wellbay and service a plurality of wells.

The cylinders may be arranged as hanging cylinders with moment free connection to the trolley structure as well as the surrounding structure, e.g., by means of shackles.

The cylinders may be arranged in close proximity to one side of the tensioning device. Since the trolley structure may be designed for large eccentric loadings this arrangement will be very robust in the case of a one cylinder failure. As the trolley rail system already has a considerable capacity in handling eccentric forces and moments, such incidents will allow continued operation, provided the system is designed for and still has sufficient tensioning capacity.

In certain embodiments, the hanging cylinders are attached to the bottom of the trolley, this allows the height of the entire system to be reduced in the same order as the trolley vertical height, which gives reduced elevation of drillfloor and associated drilling arrangements.

Additionally, there may be hatches arranged above the cylinders in each well slot, which allows them to be pulled directly up, vertically in free air by platform cranes.

The cylinders may be given a protective partitioning/housing including a sliding/sleeve portion in the lower section, which will protect against jet fires, explosions and fire in the wellbay area.

The flexible hoses may be arranged so their rolling U-shape motion compensations are arranged so the hoses may dip under the elevation of the collar/riser top.

The well slot grid may be arranged so that there is room for a drilling slot that utilizes a similar type trolley based tensioner, and a surface based BOP resting on the same tensioner.

The drilling riser may have a keel guide arrangement.

The slot may be used for a subsea BOP drilling system, and the slot may be of a size allowing use of traditional drilling tensioners (wire or in-line direct acting tensioners).

The trolley design may be fabricated in steel or other welded material where the collar pipe (“conductor”) is the main structural element secured to the rails in it upper and lower extremities.

The trolley design may have substantially all of its oil reservoir and gas accumulators located outside of the wellbay area in close proximity of the tensioning cylinders in allocated spaces that follows the pitch of the well slots. Accumulator bottles may be stored vertically or horizontally allowing short piping and straight piping to the cylinders. This will allow reduced energy losses in the system as well as increased safety and redundancy, as the system is away from the wellbay area and less likely to be impacted by any dropped object or equipment handling from the drilling activities.

Thus, the preferred embodiments have been fully described above. Although the invention has been described based upon these preferred embodiments, it would be apparent to those of skill in the art that certain modifications, variations, and alternative constructions could be made to the described embodiments within the spirit and scope of the invention. 

1. A riser tensioning system with an individual riser unit on an oil or gas platform, said riser tensioning system comprising: a traveling trolley structure having at least one trolley bearing and a centralizer; at least one guide rail coupled to said trolley bearing, configured to allow vertical movement of said traveling trolley structure; at least one cylinder attached to said traveling trolley structure on one end and secured on an opposite end, said at least one cylinder being adapted to push or pull said traveling trolley structure vertically; and a riser collar coupled to said traveling trolley structure and configured to support a top of the individual riser.
 2. The riser tensioning system recited in claim 1, wherein: said traveling trolley structure is a beam structure with two trolley bearings on opposite sides of said beam structure; said riser tensioning system has two guide rails, wherein each guide rail is coupled to a trolley bearing; and said riser tensioning system has two cylinders, wherein each cylinder is attached to the traveling trolley structure on an end and attached to said guide rail on an opposite end.
 3. The riser tensioning system recited in claim 1, wherein: said traveling trolley structure is a box structure with two trolley bearings on opposite sides of said box structure; said riser tensioning system has two guide rails, wherein each guide rail is coupled to two trolley bearings; and said riser tensioning system has two cylinders attached to the traveling trolley beam on a end and attached to said guide rail on an opposite end.
 4. The riser tensioning system recited in claim 1, wherein: said traveling trolley structure is a beam structure with two trolley bearings on opposite sides of said beam structure; said riser tensioning system has two guide rails, wherein each guide rail is coupled to a trolley bearing; and said riser tensioning system has two cylinders, wherein each cylinder is attached to the traveling trolley structure on a end and attached to a deck beam on an opposite end.
 5. The riser tensioning system recited in claim 1, wherein: said traveling trolley structure is a box structure with two trolley bearings on opposite sides of said box structure; said riser tensioning system has two guide rails, wherein each guide rail is coupled to two trolley bearings; and said riser tensioning system has two cylinders attached to the traveling trolley beam on a end and attached to a deck beam on an opposite end.
 6. The riser tensioning system recited in claim 1, wherein: said traveling trolley structure has two trolley bearings on a same side of said traveling trolley structure; said riser tensioning system has a guide rail coupled to said two trolley bearings; and said riser tensioning system has one cylinder attached to said traveling trolley structure on a end and attached said guide rail on an opposite end.
 7. The riser tensioning system recited in claim 1, wherein: said traveling trolley structure has two trolley bearings on a same side of said traveling trolley structure; said riser tensioning system has one guide rail coupled to said two trolley bearings; and said riser tensioning system has one cylinder attached to said traveling trolley structure on a end and attached to a deck beam on an opposite end.
 8. A riser tensioning system recited in claim 1, further comprising: at least one second cylinder; a wire and sheave arrangement coupled to said second cylinder on an end and coupled to the riser tensioning system on an opposite end, wherein said wire and sheave arrangement is adapted to push or pull said traveling trolley structure vertically.
 9. A method for applying tension to an individual riser unit on an oil or gas platform, said platform having a riser with a x-mas tree, a traveling trolley structure with at least one trolley bearing and a centralizer and at least one guide rail coupled to said traveling trolley structure, said method comprising the following step: attaching at least one cylinder to said traveling trolley structure and securing the other end of said cylinder, such that actuation of said cylinder effects vertical movement of said traveling trolley structure.
 10. The method recited in claim 9, wherein said attaching step comprises: attaching two cylinders on one end to said traveling trolley structure, wherein said traveling trolley structure is a beam structure, and attaching an opposite end of each said cylinder to two different guide rails.
 11. The method recited in claim 9, wherein said attaching step comprises: attaching two cylinders on one end to said traveling trolley structure, wherein said traveling trolley structure is a box structure, and attaching an opposite end of each said cylinder to two different guide rails.
 12. The method recited in claim 9, wherein said attaching step comprises: attaching two cylinders on one end to said traveling trolley structure, wherein said traveling trolley structure is a beam structure, and attaching an opposite end of each said cylinder to a deck beam.
 13. The method recited in claim 9, wherein said attaching step comprises: attaching two cylinders on one end to said traveling trolley structure, wherein said traveling trolley structure is a box structure, and attaching an opposite end of each said cylinder to a deck beam.
 14. The method recited in claim 9, wherein said attaching step comprises: attaching a cylinder on one end to said traveling trolley structure and attaching an opposite end of each said cylinder to said guard rail.
 15. The method recited in claim 9, wherein said attaching step comprises: attaching a cylinder on one end to said traveling trolley structure and attaching an opposite end of each said cylinder to a deck beam.
 16. The method recited in claim 9, further comprising the following steps: attaching at least one second cylinder to a wire and sheave arrangement and securing the other end of said cylinder, such that actuation of said cylinder effects vertical movement of said traveling trolley structure.
 17. A riser tensioning system with an individual riser unit on an oil or gas platform, said platform having a traveling trolley structure with at least one trolley bearing and a centralizer, said trolley bearing coupled to at least one guide rail configured to allow vertical movement of said traveling trolley structure, said traveling trolley structure coupled to a riser collar configured to support a top of said riser, said riser tensioning system comprising: at least one cylinder attached to the traveling trolley structure on one end and secured on an opposite end such that said cylinder is adapted to push or pull said traveling trolley structure vertically.
 18. The riser tensioning system recited in claim 17, wherein: said riser tensioning system has two cylinders, wherein each cylinder is attached on an end to the traveling trolley structure, wherein the traveling trolley structure is a beam structure, and attached on an opposite end to a bottom of the guide rail.
 19. The riser tensioning system recited in claim 17, wherein: said riser tensioning system has two cylinders, wherein each cylinder is attached on an end to the traveling trolley structure, wherein the traveling trolley structure is a box structure, and attached on an opposite end to a bottom of the guide rail.
 20. The riser tensioning system recited in claim 17, wherein: said riser tensioning system has two cylinders, wherein each cylinder is attached on an end to the traveling trolley structure, wherein the traveling trolley structure is a beam structure, and attached on an opposite end to a deck beam.
 21. The riser tensioning system recited in claim 17, wherein: said riser tensioning system has two cylinders, wherein each cylinder is attached on an end to the traveling trolley structure, wherein the traveling trolley structure is a box structure, and attached on an opposite end to a deck beam.
 22. The riser tensioning system recited in claim 17, wherein: said riser tensioning system has one cylinders, wherein each cylinder is attached on an end to the traveling trolley structure and attached on an opposite end to said guard rail.
 23. The riser tensioning system recited in claim 17, wherein: said riser tensioning system has one cylinders, wherein each cylinder is attached on an end to the traveling trolley structure and attached on an opposite end to a deck beam.
 24. The riser tensioning system recited in claim 17, further comprising: at least one second cylinder; a wire and sheave arrangement coupled to said second cylinder on an end and coupled to the riser tensioning system on an opposite end, wherein said wire and sheave arrangement is adapted to push or pull said traveling trolley structure vertically. 